1. Field of the Invention
The present invention relates to a luminescent element having an organic material layer composed of a luminescent material in which recombination energy of injected charge is directly converted into optical energy by an applied electric field.
More particularly, the invention relates to a luminescent element, unlike conventional incandescent lamps, fluorescent lamps, light emitting diodes, etc., used in organic light emitting diode (OLED) panels which are thin, lightweight, solid-state devices having a large area and high resolution, and are capable of high-speed operation, thus satisfying advanced requirements.
2. Description of the Related Art
Pope et al., observed electroluminescence (EL) in an organic material, namely, single-crystal anthracene, in 1963 (J. Chem. Phys., 38, 2042 (1963)). Subsequently, Helfinch and Schneider successfully observed relatively strong EL in an injection-type luminescent element using a solution electrode system having high injection efficiency (Phys. Rev. Lett., 14, 229 (1965)).
Many studies of organic luminescent materials containing conjugated organic hosts and conjugated organic activators having condensed benzene rings have been conducted, such as those disclosed in U.S. Pat. Nos. 3,172,862, 3,173,050, and 3,710,167; J. Chem. Phys., 44, 2902 (1966); J. Chem. Phys., 50, 14364 (1969); J. Chem. Phys., 58, 1542 (1973); and Chem. Phys. Lett., 36, 345 (1975). Examples of disclosed organic hosts include naphthalene, anthracene, phenanthrene, tetracene, pyrene, benzopyrene, chrysene, picene, carbazole, fluorene, biphenyl, terphenyl, triphenylene oxide, dihalobiphenyl, trans-stilbene, and 1,4-diphenylbutadiene. Examples of disclosed activators include anthracene, tetracene and pentacene. Since these organic luminescent materials are provided as single layers having a thickness of more than 1 xcexcm, a high electric field is required for luminescence.
Under these circumstances, thin film elements formed by a vacuum deposition process have been proposed (for example, Thin Solid Films, 94, 171 (1982); Polymer, 24, 748 (1983); and Jpn. J. Appl. Phys., 25, L773 (1986)). Although the thin film elements are effective in reducing the driving voltage, their luminance level is not sufficient for practical use.
Recently, Tang et al., have developed a luminescent element having a high luminance at a low driving voltage (Appl. Phys. Lett., 51, 913 (1987) and U.S. Pat. No. 4,356,429). The luminescent element is fabricated by depositing two extremely thin layers, namely, a charge-transporting layer and a luminescent layer, between an anode and a cathode by a vacuum deposition process. Layered organic LED devices of this type are also disclosed in, for example, Japanese Patent Laid-Open Nos. 59-194393, 63-264692, and 3-163188, U.S. Pat. Nos. 4,539,507 and 4,720,432, and Appl. Phys. Lett., 55, 1467 (1989).
Also, an organic LED element of a triple-layered structure in which a carrier transport ability and a luminescent ability are separately provided is disclosed in Jpn. J. Appl. Phys., 27, L269 and L713 (1988). Since the restrictions on the selection of dyes in the luminescent layer due to the carrier transportability are relieved in such a luminescent element, the selection of possible dyes in the luminescent layer is considerably increased. Furthermore, the device configuration suggests the possibility of improved luminescence by effectively trapping holes and electrons (or excitons) in the central luminescent layer.
Layered organic LED elements are generally formed by vacuum deposition processes. Luminescent elements having considerably high luminance are also formed by casting processes (as described in, for example, Extended Abstracts of the 50th Meeting (1989), p. 1006, and the 51st Meeting (1990), p. 1041, of The Japan Society of Applied Physics).
Moreover, considerably high light emission efficiency is also achieved by a single-layered mixture-type organic LED device, in which the layer is formed by immersion-coating of a solution containing polyvinyl carbazole as a hole-transporting compound, an oxadiazole derivative as an electron-transporting compound and coumarin-6 as a luminescent material, as described in Extended Abstracts of the 38th Meeting 1991, p. 1086, of The Japan Society of Applied Physics and Related Societies.
As described above, organic LED devices have been significantly improved, and applications thereof to broad areas are expected.
However, materials and fabrication methods for the organic LED devices have not yet been thoroughly researched. There are still problems with durability, such as changes over time due to light emission of higher luminance and use for a long period of time, and deterioration due to atmospheric gas which is humid and contains oxygen.
For example, fluorescent organic solids which are used as materials for luminescent layers are easily affected by moisture, oxygen, etc. Electrodes formed on luminescent layers directly or with hole-injecting layers or electron-injecting layers therebetween are easily degraded by oxidation. Consequently, when conventional organic LED elements are operated in air, luminous characteristics rapidly degrade. Therefore, in order to obtain practical organic LED elements and organic LED devices, the elements must be sealed in order to prevent the luminescent layers from being affected by moisture, oxygen, etc. and so that electrodes are not oxidized, thereby prolonging life.
However, an effective sealing method for organic LED elements has not yet been developed. For example, when a method for sealing an inorganic EL element is used for an organic LED element, that is, when a back glass plate is provided on the exterior of a back electrode and a silicone oil is enclosed between the back electrode and the back glass plate, the silicone oil infiltrates into the luminescent layer through the electrode, or through the electrode and a hole-injecting layer or an electron-injecting layer, and the luminescent layer is altered by the silicone oil, and thus the luminous characteristics of the organic LED element are greatly degraded or the light emission capability is completely lost.
Additionally, when a resin coating layer provided for mechanical protection and the like is used for sealing an organic LED element, since certain types of resin coating solutions dissolve the luminescent layer as described above, luminous characteristics of the organic LED element are greatly degraded or the light emission capability is completely lost.
Japanese Patent Laid-Open No. 5-21159 discloses a structure in which an insulating layer containing barium titanate and a polyamide as a hygroscopic material is provided between an electrode and a luminescent layer, and Japanese Patent Laid-Open No. 6-119970 discloses a structure in which an insulating layer containing barium titanate and a conductive water-capturing layer are provided between a luminescent layer and an electrode. However, if such an insulating layer is provided between an organic material layer and an electrode in a charge-injection-type organic luminescent element, luminance may be decreased.
Although various sealing techniques have been attempted as disclosed in Japanese Patent Laid-Open Nos. 2-260388, 3-261091, 4-137483, 4-212284, 5-36475, 5-89959, 5-101885, 5-335080, 6-96858, 6-176867, 7-14675, 7-147189, 7-161474, 7-169569, 7-192868, 8-78159, 8-96955, 8-96962, 8-111286, 8-185982, 9-153395, 9-204981, 9-245964, 10-275680, etc., the foregoing problem has not yet been sufficiently solved.
Additionally, luminescence of an organic LED element is caused by recombination of charges (electrons and holes) injected from electrodes. However, mobility of charge is greatly inhibited by traps and the like in the organic material layer, and the amount of charges (electrons and holes) injected from the electrodes becomes unbalanced, resulting in a decrease in luminance.
Therefore, to enable stable operation of the element and to prolong the element life, the space-charge effect in the element must be improved.
Accordingly, it is an object of the present invention to provide a luminescent element having a long life and a stable optical output, and in which moisture, oxygen, etc., are prevented from entering an organic material layer in the element, and space charges in the element are overcome.
In accordance with the present invention, an organic luminescent element includes a substrate, an anode, a cathode, at least one organic material layer interposed between the anode and the cathode, and a sealing film containing a ferroelectric material provided on the anode or the cathode disposed on the organic material layer.
Preferably, the organic luminescent element further includes a counter electrode provided on the sealing film. Preferably, the sealing film is provided on the cathode, and an electric potential higher than that of the cathode, more preferably, a positive electric potential is applied to the counter electrode. Alternatively, preferably, the sealing film is provided on the anode, and an electric potential lower than that of the anode, more preferably, a negative electric potential is applied to the counter electrode.
Preferably, the substrate is transparent, and the anode contains indium tin oxide (ITO).
The counter electrode may be connected to a poling power supply. Alternatively, the counter electrode may have the same electric potential as that of the cathode when the sealing film is provided on the anode, and the counter electrode may have the same electric potential as that of the anode when the sealing film is provided on the cathode.
Preferably, the organic luminescent element emits light by DC driving or pulsed driving.
Combinations of the above features are also in the scope of the present invention.
In accordance with the present invention, space charges accumulated in the organic material layer of the element are diffused, and it is possible to provide a luminescent element which can be stably operated for a long period of time.
Further objects, features and advantages of the present invention will be apparent from the following description of the preferred embodiments with reference to the attached drawings.